Introduction

The main aim of this thesis is to recover, calibrate and interpret non-prime data recorded with the ISO Long Wavelength Spectrometer (LWS) in its Fabry-Pérot (FP) mode. This chapter describes the operation of the instrument and defines both prime and non-prime data. The giant molecular cloud, Sagittarius B2 (Sgr B2), was observed in a complete unbiased spectral survey described in Section 1.9 and these data are then investigated in detail in the remaining chapters.

A review of the Sgr B2 source and its line of sight are presented in Chapter 2. The LWS observations of Sgr B2 are then introduced in Chapter 3. These consist of the full spectral survey carried out in the LWS L03 mode as well as individual narrow FP observations and several grating mode measurements. Improvements made to the flux calibration of FP data are then discussed. These improvements were derived from both prime and non-prime observations in the Sgr B2 L03 dataset. The major achievements presented in this chapter are the calculation of accurate dark currents which include the contribution of stray light, an algorithm to correct for the drift in detector responsivity, the recovery of accurate grating profile shapes and the development of a correction to account for FP side order contamination. These resulted in new calibration files and reduction algorithms that were implemented in a revised version of the standard LWS Interactive Analysis (LIA) software. This was released to the astronomical community as LIA version 10 in November 2001.

Chapter 4 gives an overview of the improvements in the reduced data that were achieved using the new software. The full spectrum in prime observations is presented with a list of assigned and unassigned features. Specific improvements using non-prime data are then outlined.

The remaining chapters detail the analysis carried out on specific spectral lines from the survey. Chapter 5 describes the forbidden lines of atomic oxygen, used to probe the structure of the clouds along the line of sight. The [OI] 63 $\mu$m line was observed in absorption, indicating that a large proportion of the oxygen along the line of sight is in atomic form. This chapter also gives a detailed account of the data reduction and error budget involved in the analysis of L03 spectral lines. Chapter 6 relates to the detection of the HD $J=1\rightarrow0$ rotational transition in emission. This line was used with an upper limit for the $J=2\rightarrow1$ transition to determine a limit on the temperature in the emission region. The corresponding column density of H$_{2}$ was estimated and this allowed the deuterium abundance in the Sgr B2 envelope to be calculated. This is one of the few direct measurements of the deuterium abundance that have been carried out in the Galaxy further than 500 pc from the Sun. Chapter 7 details the analysis of OH lines in the LWS spectral range. Rotational lines were detected due to OH containing all three stable isotopes of oxygen. In the case of $^{17}$OH this was the first detection of these lines in the ISM. These lines were modelled accounting for the line of sight clouds and used to investigate the variation in oxygen isotope ratios through the Galaxy. The final chapter presents overall conclusions and future work.